1. Technical Field of the Invention
The present invention relates to a plasma display panel (PDP). In particular, the present invention relates to the PDP, wherein power dissipation is reduced, display luminance is increased, and gradation is displayed smoothly by controlling the number and period of pulses for sustaining discharge (SD) in plasma.
2. Description of the Prior Art
In the plasma display, fluorescent substance is exited by ultraviolet light generated by gaseous discharge, thereby making the fluorescent substance emit light. The plasma display is applied to large scene TV display and information display.
A PDP structure as shown in FIG. 10A is a top view of a back substrate with a part broken away. FIG. 10B is a sectional view of a front substrate, and FIG. 10C is a sectional view of the back substrate. Front substrate 100 acting as a display surface has a plurality of transparent electrodes 3 and a plurality of narrow-width bus electrodes 4 formed in parallel on glass substrate 1. Thereon, dielectric layer 7 and protection layer 8 are formed. As dielectric layer 7, low melting point glass is typically used. Magnesium oxide thin film, for example, is used for protection layer 8, because its secondary electron emission factor is sufficient, and it is not easily sputtered by the plasma gas. On glass substrate 2, data electrodes 5 are formed, and then dielectric, layer 10 and partitions 6 are formed. Powdered fluorescent substances 9 for R,G,B display are successively coated on bottom faces and side faces of grooves formed by the partitions. Thus, back substrate 200 is thus completed. The partitions ensures discharge spaces, and also have an effect of preventing discharge crosstalk and blurring of displayed color. Back substrate 200 and front substrate 100 are combined. Surroundings of both the substrates are sealed by frit glass. Finally, discharge gas having inert gas as its principal ingredient is introduced to complete PDP apparatus. The transparent electrodes with bus electrodes on front substrate 100 are paired. The gap between them becomes planar discharge gap 11. One of them is used as scan electrode 12, and the other of them is used as sustaining electrode 13. Data electrode 5, scan electrode 12 and sustaining electrode 13 are driven by various voltage waveforms.
FIG. 11 shows an example of a basic driving waveform of an AC planar discharge type PDP. In the scheme shown in FIG. 11, driving is conducted separately in write interval 23 including preparation interval 21 and scan interval 22, and sustaining interval 26 including sustaining discharge interval 24 and blanking interval 25. Scan pulses are applied successively to scan electrodes. According to this timing, a data pulse having a polarity opposite to that of the scan pulse is applied to the data electrode on the basis of display data for a display cell on the scan electrode. As a result, opposite discharge in the direction perpendicular to the substrates is generated between the scan electrode and the data electrode. Further, this opposite discharge serves a trigger, and planar discharge in the direction parallel to the substrate surface is generated between the sustaining electrode and the scan electrode. Write operation is thus completed. By this write discharge, wall charge is formed on the surface over the scan electrode and the sustaining electrode. In a cell having wall charge formed therein, sustaining discharge of planar discharge is generated by a sustaining pulse applied between the sustaining electrode and the scan electrode. In a cell for which writing has not been conducted, however, sustaining discharge is not generated even if a sustaining pulse is applied, because there is no superposition effect of the electric field brought by wall charge. By applying the sustaining pulses a predetermined times, light emission display is conducted. By the way, in order to enhance the write operation performance, pre-discharge operation, wherein a high voltage is applied to all cells prior to the write operation to erase the past record of the display state and forcibly cause discharge or the like is adopted. As heretofore described, the plasma display driving sequence is formed by a sequence of the preparation operation, write operation, and sustaining light emitting operation.
Although the write interval and the sustaining interval are separated in the above explanation, these intervals may be overlapped partially under the condition that the write operation is disposed after the preparation operation, and thereafter the sustaining operation is disposed.
The sub-field method is used for displaying gradation. Because voltage modulation of the light emitting display luminance is difficult in an AC type plasma display device, and it is necessary to change the number of times of light emission for luminance modulation. The sub-field method includes the steps of resolving one image having gradations into a plurality of binary display images, displaying the binary display images consecutively at high speed, and reproducing a multi-step gradation image by using a visual integration effect.
FIG. 12 shows an example, wherein an image of 256 step gradation is represented by 8 sub-fields. Image luminance signal data is digitized by using a binary code having ratios of 128: 64: 32: 16: 8: 4: 2: 1, and sub-fields having a number of sustaining pulses which provide luminance values corresponding to the respective step of gradation are assigned. The number of sustaining pulses of each sub-field is adjusted so that a top sub-field SF1 supplies a maximum luminance, SF2 supplies a luminance equivalent to half of the luminance of SF1, and SF8 supplies a luminance of the lowest order. Sub-fields are selected, in accordance with the gradation level of each discharge cell.
However, spurious image occurs in moving pictures in the PDP. Therefore, redundant codes are added for the prevention of the spurious image. Accordingly, for example, more than nine sub-fields are required for 256 steps gradation display. As a result, the sustaining interval directly contributing to the light emission must be shortened.
The light emission efficiency of plasma display is not so high. In the case of bright display over the whole panel serface, such as the whole white, therefore, electric power consumption is increased, resulting in a problem of power dissipation and a problem of heat generation of the panel and the circuit. Therefore, when the average luminance of the panel surface is low, it is necessary to fall down the whole white luminance and at the same time to raise the peak luminance, when the average luminance of the panel surface is low. Concretely, an average luminance level of the whole panel surface (APL) is detected, and the number of the sustaining discharge pulses of each sub-field is changed according thereto. When the APL is low, the number of sustaining discharge pulses is increased. On the other hand, when the APL is high, the number of sustaining discharge pulses is decreased.
FIG. 13 schematically shows the relation between the APL level and the number of sustaining pulses. In this example, Eight steps are used for the APL level. The lowest level is provided with APL level 0, whereas a state near the whole white state is provided with APL level 7. The ordinate represents the number of sustaining pulses per frame and indicates the number of sustaining pulses at each APL level. In the whole white state, the number of sustaining pulses is 510 even at the luminance level 255 which is the highest luminance level. At the APL level 0 providing a peak luminance, however, the number of sustaining pulses is 1020 at the luminance level 255. Namely, as many sustaining pulses as twice the number of sustaining pulses for the whole white display are applied. A peak luminance which is approximately twice the whole white luminance is implemented. For reference, the number of sustaining pulses at the gradation level 127 in each APL level is also shown in FIG. 13.
The power consumption becomes maximum at the time of the whole white display. Without causing an increase of the maximum power dissipation, it is possible to attempt to increase the peak luminance when the APL level is small. As for the APL detection, there are various methods. In the case of plasma display, however, luminance data is handled by a digital signal and the APL level can be detected easily by simple digital signal processing. Further, setting the number of sustaining pulses of each sub-field corresponding to each APL level can be conducted simply by using, for example, a look-up table (LUT) . In the example of FIG. 13, the number of steps of the APL control has been set to be 8 for simplicity. In order to make brightness changes at transitions between the steps smoother, however, further more gradation steps are being employed for the PDP in practical use.
The method of controlling the number of sustaining pulses with information corresponding to the APL level for reducing the maximum power dissipation and for increasing the peak luminance as described above is called power saving method or peak luminance enhancing (PLE) method.
Although above described PLE method is useful for the plasma display, but it is still insufficient as compared with CRT. Therefore, further improvement thereof is desired. For example, the write interval in the PDP in practical use must be long, particularly, for achieving higher gradations, improved quality of moving pictures, and large capacity display. In this case, the sustaining interval directly relating to the light emission luminance must be further shortened.
As a result, there is a disadvantage that the peak luminance can not be made sufficiently high by the conventional PDP. If the repetition period of the sustaining pulses is shortened, it is possible to apply many sustaining pulses during the sustaining interval, thereby raising the display luminance. In this case, however, the light emission efficiency is lowered due to the saturation phenomenon of the fluorescent substance and ultraviolet light emission. Therefore, the increased power consumption does not pay the increase in the panel luminance.
Further, the luminance ratio PL/WL of the peak luminance PL to the whole white luminance WL can not be made so high. The ratio PL/WL is typically 2 to 3, and it is low as compared with CRT. This is because the sustaining interval for defining the peak luminance can not be made sufficiently long. Further, the gradation can not be displayed smoothly, when the peak luminance of 400 cd/m2 with 256 step gradation for PDP is required. Here, 400 cd/m2 luminance is easily obtained on the CRT. This is because one gradation level corresponds to 1.5 cd/m2 in. Therefore, dark scenes become unnatural on PDP. On the other hand, even when the peak luminance is high on the CRT, the smoothness of the gradation is not hampered, because of the analogue expression of gradation. Therefore, conventional PDP has another disadvantage that the gradation is not displayed smoothly, compared with the CRT. This is because the PDP displays the gradation by using digital signal.